Structures and Materials

Question 1

Airframe

Answer 1

The aircraft without equipment and furnishment; The skin and frameworkd that provide aerodynamic shape; The load bearing parts; The parts that together protect the content from the environment

Question 2

Primary and secondary structures

Answer 2

Primary structures are critical components, whose damage or failure could lead to failure of the entire aircraft. Secondary structures support aerodynamics, safety, comfort, but are not essential

Question 3

Truss structures

Answer 3

Structures made from bars, tubes, wires; structural rigidity is obtained by placing elements in the diagonal (they can withstand compression or tension); later sheets were used for the load bearing function; Truss structures are still used today for weight saving;

Question 4

Sheet stiffening

Answer 4

Geometrically manipulating sheet in order to carry compressive loads e.g. sheet profiles;

Question 5

Shell structure

Answer 5

Sheet reinforced by a stiffener

Question 6

L-shaped stiffener

Answer 6

Stiffener in the form of L; will bulge out more easily; can be easily inspected; lighter;

Question 7

Hat stiffener

Answer 7

In the form of a parallelogram; attached to the sheet; very strong; can not be inspected on the inside; heavier;

Question 8

Distance between stiffeners

Answer 8

A single stiffener has only local effect, thus optimal spacing between stiffeners shall be chosen

Question 9

Thought about compression of sheets

Answer 9

Sheets actually do have compressive resistance. The problem is that they bulge before showing the resistance. Thus when reinforced, they do not bulge and actually contribute to compressive resistance.

Question 10

Sandwich structure

Answer 10

Low-density core polymer core (usually honeycomb structure) with two thin alimunim or carbon fibre reinforced polymer sheets.

Question 11

Polymer

Answer 11

A repeating chain or branch of molecular structure (monomer). Usually very lightweight, durable, flexible.

Question 12

Advantages of sandwich structures

Answer 12

High strength-to-weight ratio. Good rigidity to weight. Absorbing impact energy. Abosrbing vibrations. Thermal and Acoustic insulation. Very high bending stifness.

Question 13

Disadvantages of sandwich structures

Answer 13

Trap moisture which leads to corrosion in metals. Also moisture could freeze, then exapnds and delaminates the core form the sheets. Also moisture adds weight. Complex mechanical joints, due to the soft core which can not carry loads. Also the out of plane stiffness is very low, so no bolted joints.

Question 14

Integrally stiffened structures

Answer 14

Structures that are made of thick sheet of metal with mechanically machined geometry.

Question 15

Pros of integrally stiffened structures

Answer 15

The process is automated. Low amount of parts. The thickness could be regulated.

Question 16

Cons of integrally stiffened structures

Answer 16

A lot of scrap metal (but nowadays parts come pre-miled and the supplier could re-use the metal). Complex stiffener concepts cannot be applied. Has no natural barriers (bolting or riveting) for cracks. Therefore, less damage-tolerant.

Question 17

Fuselage

Answer 17

A thin-wall pressure vessel exploiting the stifenned shell concept

Question 18

Fuselage skin

Answer 18

The outermost layer of the fuselage. Protects the content from the environment.

Question 19

Frames

Answer 19

The are in the cross section of the fuselage skin. Provide and separate the cylindrical shape into structures and provid lateral stifness.

Question 20

Stringers

Answer 20

Long stiffened elements providing the longtitudial stifness of an aircraft

Question 21

Bulkheads

Answer 21

Separate the aircraft into sections. Abosrb impact forces. Keep the cabin pressurized. Absorb pressurization forces.

Question 22

Splices and joints

Answer 22

Provide the mechanical connections between stringers, frames and fuselage skin.

Question 23

Division of wing types

Answer 23

Kink or no kink in wing spars. Ribs parallel to flight direction or perpendicular to spars.

Question 24

Disadvantage of sectioning large wings into sections

Answer 24

Highly loadead joints (rainbow fitting)

Question 25

Functions of ribs

Answer 25

Maintain aerodynamic profile; transfer aerodynamic and fuel weight loads into the rest of the wing structure; provide stability against panel crushing and buckling; introducing local loads (landing gear, engines, flaps); sealing in case of integral tanks (fuel does not splash in the whole wing);

Question 26

Desing of ribs

Answer 26

Depends on local loads, desing philosophy (allowed magnitude of stress), available equipment and experience, costs; low loads -> forming sheet or plate material into a rib; high loads -> forging or machining from a thick plate;

Question 27

Spars

Answer 27

Usually I-beams which go spanwise the wing and provide the bending stifness; Must be thicker near root tip due to larger forces; Due to extrusion process varying cross-sections impossible, thus additional stiffeners (reinforcement) are added via riveting or bolting;

Question 28

Differential bending

Answer 28

A single spar cannot withstand torsion. Therefore, we use differential bending (two spars) and the torsion is translated into two opposite bendings

Question 29

Torsion box

Answer 29

A closed box, which can carry out torsional loads. The strength is provided by the shear resistance of the infinitesimal elements of the structure. Thicker skin for greater strength.

Question 30

Advantages of torsion box

Answer 30

We can create longer wings for the same thickness. Also torsion stiffness and bending strength can be engineered seperately

Question 31

Doublers

Answer 31

Due to the non-constant loads throughout the structure varying thickness is necessary. Thus second layer is added (doublers). Alternatively, thickness step is machined.

Question 32

Joggling

Answer 32

To follow the thickness step stringer are joggled (shear deformation along the geometry of surface)

Question 33

Splices

Answer 33

Used to connect different parts of the airframe (skin panels, stringers). Three tipes lap (overlapping elements), butt (next to each other connected via doubler), scarf (forming one into the other)

Question 34

Stringer Frame connection

Answer 34

Two main methods, in both of them the stringer is uninterrupted. Either we interrupt the frame (local weakening) or we position it above the stringer.

Question 35

Payload adaptor

Answer 35

Inserts the load provided by the launcher into the structure; Primariliy done by strut structures; main function is to insert load via compression, thus can be optimized for this (Space shuttle ribs)

Question 36

Central cylindrical structure

Answer 36

A structure with a large cylindrical thrust-load-bearing member; it is used to transfer the loads from the thrust throught the entire structure up to the adaptor; all systems are attached to strong points via struts, platforms and shear webs;

Question 37

Payload fairings

Answer 37

protects the payload from the agressive environment (heating and pressure) during launch; once outside the atmosphere the fairings are disposed; the accelerated mass is decreased, thus efficiency of thrust is increased;

Question 38

Stage structures

Answer 38

Rockets are divided into stages; each has a propulsion system and subsystems; main stage forms the core; boosters are usually parallel to the main stage;

Question 39

Stage structure design concepts

Answer 39

Either the fuel tanks are part of the load-bearing structure or they are not. If they are the different components are connected via interface structures to transfer loads between components and skin concept is semi-monocoque; If they are not, there is an external skin seperated by the fuel tanks via longerons and circular stiffeners;

Question 40

Thrust structures

Answer 40

Extremely high loads (Ariane 5, 1500 tons); Very concentrated, thus introduces with conical structure;